This application claims priority of Taiwanese application no. 091112415, filed on Jun. 7, 2002.
1. Field of the Invention
The invention relates to an EFM/EFM+ encoding system, more particularly to a method and apparatus that enable an EFM/EFM+ modulated bit sequence to have a relatively small Digital Sum Value (DSV), and to the inhibiting of unauthorized copying of data by introducing special data patterns which cause a relatively large DSV.
2. Description of the Related Art
Generally, digital data is seldom transmitted or recorded directly in its original digital form. Prior to transmission or recording of digital data, the digital data is usually converted to another data pattern through a certain modulation method. In compact disc (CD) or digital versatile disc (DVD) recording systems, the data to be recorded on a compact disc is modulated using EFM (eight-to-fourteen modulation), whereas the data to be recorded on a digital versatile disc is modulated using EFM+ (eight-to-fourteen modulation plus). However, during the process of EFM or EFM+ modulation, in case there exist certain special data patterns in the modulated data, these special data patterns will cause the modulated bit sequence to produce a very large variance of DSV (Digital Sum Value). For instance, referring to FIG. 1, when EFM is used to modulate a special data pattern of {0x9a, 0xb9, 0x9a, 0xb9, 0x9a, 0xb9, . . . } (hexadecimal form), it is noted that the absolute value of DSV generated from the channel bits after modulation will cumulatively increase and cannot be controlled via the standard EFM modulation. If the value of DSV cannot be controlled to be small, the excessively large DSV of the recorded data results in that the data slicer, conventionally used to retrieve the binary signal from the analog signal detected on optical discs, cannot function correctly, and the data readout from the disc is erroneous. Moreover, the large DSV variance implies that the EFM signal is no longer DC-free, i.e., free of direct current (DC) components, and the low-frequency components of the EFM signal interfere with the related servo control signal of the optical disc system. Therefore, to solve such a problem of EFM/EFM+ modulation, the current CD/DVD specifications provide a data scrambling mechanism, which re-arranges the original data to prevent emergence of the aforesaid special data pattern, to avoid occurrence of such a situation (i.e., excessively large DSV). EFM+ is a state-dependent modulation, and the modulation proceeds according to the state determined by the preceding symbol and the current symbol, which ensures that the adjacent codewords, when connected, can comply with the run length limit (RLL) of 3xcx9c11T even without the need for a merging bits mechanism used in EFM modulation. In EFM+ modulation, more than one codeword maybe available for a data symbol in some cases, and the codeword with the smallest DSV is selected as the modulation result. However, if a special data pattern, such as {94, 252, 231, 231, 231, 231, . . . } is to be modulated using EFM+, the situation that DSV cumulatively increases will occur, as shown in FIG. 2. Besides, in the DVD system, a xe2x80x9cSYNC-code reselection algorithmxe2x80x9d has been proposed heretofore to limit the largest variance of DSV and prevent the divergence of DSV. Although the SYNC-code reselection algorithm provided by EFM+ can prevent undesired DSV divergence, a very large DSV (around xc2x1300xcx9c400) will still be generated. When DSV is very large, the modulated EFM signal is not DC-free, and the conventional data slicer used to retrieve data recorded on optical discs cannot work correctly. Therefore, the mechanism in the aforesaid CD/DVD specifications can only reduce the possibility of appearance of these special data patterns and limit the damage caused thereby. These special data patterns may still occasionally appear in general data and can hinder reading of data recorded on discs.
In addition, a number of disc copying protection methods have been proposed heretofore using the aforesaid special data patterns. For instance, U.S. Pat. No. 5,828,754 discloses a method of inhibiting copying of digital data in a digital recording system, such as DVD-ROM, in which special data patterns are added into source data before error correcting code (ECC) encoding so that ECC encoded data causes large DSV variance after EFM/EFM+ modulation. In view of the designed large DSV variance, certain deliberate errors are added into a sequence of ECC encoded symbols to interrupt the continuity of the special patterns leading to DSV progression. Subsequently, the final data is recorded on a disc, and this disc is called an authorized disc. The source data on an authorized disc can be read out correctly, because the deliberate errors can prevent the DSV accumulation generated by the special patterns and can be corrected after ECC decoding. However, once the source data is read out from an authorized disc and is reproduced on a recorded disc via the standard encoding process, the source data on the newly reproduced disc cannot be read back. This is because the deliberate errors for preventing the DSV accumulation are not generated in the standard encoding process, thereby achieving the function of inhibiting disc copying. However, the method according to the aforesaid patent suffers the following drawbacks:
1. The deliberate errors, added into the data prior to EFM/EFM+ modulation so as to avoid the excessively large DSV, require correction by the ECC in the CD/DVD. In other words, this method sacrifices a part of the capability of the ECC, and diminishes the tolerance of the optical disc with regard to other errors.
2. As the processed special symbols are selected by the system itself, it is easy to find out ways of writing error symbols in order to control DSV progression. However, the aforesaid patent is silent in regard to how to control the DSV variance of random special symbols that are not generated by the system itself.
3. In the aforesaid patent, the symbol sequence is processed further after ECC encoding, which is very inconvenient in actual use. For instance, if an entry of data is to be recorded on a disc, and the method disclosed in the aforesaid patent is used to prevent DSV progression or copying of the disc, ECC encoding of the entry of data has to be done on the computer first, followed by calculation of the DSV of the entry of data after EFM/EFM+ modulation thereof, and substitution of certain symbols. Finally, the processed symbol sequence is recorded on the disc using a recorder. Such complicated operations cannot be performed on structures of current CD/DVD recording devices to achieve real time prevention of DSV progression or inhibit disc copying.
Therefore, the main object of the present invention is to provide a method and apparatus for performing DSV protection in an EFM/EFM+ encoding system to ensure that an EFM/EFM+ modulated bit sequence has a relatively small Digital Sum Value (DSV) so as to facilitate reading.
Another object of the present invention is to provide a method and apparatus for performing DSV protection in an EFM/EFM+ encoding system that can inhibit unauthorized disc copying by introducing special data patterns into the data before performing DSV protection on the data.
According to one aspect of the invention, there is provided a method for performing Digital Sum Value (DSV) protection in an Eight-to-Fourteen/Eight-to-Fourteen Plus (EFM/EFM+) encoding system. The EFM/EFM+ encoding system is adapted to modulate a source symbol sequence into a modulated bit sequence that is further converted to a channel bit sequence, which is to be recorded on a recording medium and which is associated with a cumulative DSV, through Non-Return-to-Zero-Inverse (NRZI) conversion. The method comprises the steps of:
a) calculating the DSV associated with the EFM/EFM+ modulated bit sequence; and
b) according to the DSV calculated in step a), adjusting at least a bit in the modulated bit sequence such that the channel bit sequence from the NRZI conversion of the modulated bit sequence accumulates a relatively small DSV.
According to another aspect of the invention, there is provided a method of using an Eight-to-Fourteen/Eight-to-Fourteen Plus (EFM/EFM+) encoding system to inhibit disc copying. The method comprises the steps of:
a) finding a special data pattern that will generate a large Digital Sum Value (DSV) during modulation in the EFM/EFM+ encoding system;
b) adding the special data pattern to data that is to be recorded;
c) performing error correcting code (ECC) encoding upon the data obtained in step b);
d) performing EFM/EFM+ modulation of the data, obtained in step c) using the EFM/EFM+ encoding system to result in a modulated bit sequence;
e) calculating the DSV associated with the EFM/EFM+ modulated bit sequence; and
f) according to the DSV calculated in step e) adjusting at least a bit in the modulated bit sequence such that a channel bit sequence from subsequent Non-Return-to-Zero-Inverse (NRZI) conversion of the modulated bit sequence accumulates a relatively small DSV.
According to still another aspect of the invention, there is provided an apparatus for performing Digital Sum Value (DSV) protection in an Eight-to-Fourteen/Eight-to-Fourteen Plus (EFM/EFM+) encoding system. The EFM/EFM+ encoding system is adapted to modulate a source symbol sequence into a modulated bit sequence that is further converted to a channel bit sequence, which is to be recorded on a recording medium and which has a cumulative DSV, through Non-Return-to-Zero-Inverse (NRZI) conversion. The apparatus comprises a DSV calculation unit adapted to calculate the DSV associated with the EFM/EFM+ modulated bit sequence, and a decision unit coupled to the DSV calculation unit and adapted to adjust at least a bit in the modulated bit sequence according to the result calculated by the DSV calculation unit such that the channel bit sequence from the NRZI conversion of the adjusted modulated bit sequence accumulates a relatively small DSV.
According to yet another aspect of the invention, an Eight-to-Fourteen/Eight-to-Fourteen Plus (EFM/EFM+) encoding system capable of Digital Sum Value (DSV) protection comprises:
an EFM/EFM+ modulating device adapted to modulate a source symbol sequence into a modulated bit sequence that is further converted to a channel bit sequence, which is to be recorded on a recording medium and which is associated with a cumulative DSV, through Non-Return-to-Zero (NRZI) conversion;
a DSV calculation unit coupled to the EFM/EFM+ modulating device for calculating the DSV associated with the EFM/EFM+ modulated bit sequence; and
a decision unit coupled to the DSV calculation unit and the EFM/EFM+ modulating device for adjusting at least a bit in the modulated bit sequence according to the result calculated by the DSV calculation unit such that the channel bit sequence from the NRZI conversion of the adjusted modulated bit sequence accumulates a relatively small DSV.